There's probably two ways to think about railroading.
1) It's something that the GM does, either deliberately or through enacting bad design from an adventure module. In this case it generally means either never offering the players meaningful decisions or invalidating their decisions by always leading to the same result
2) It's the player experience and frustration of having their decisions invalidated, and not matter.
These sound like the same thing but they're not. In the case of 2) railroading is pretty much bad by definition, it's a negative experience. However, the situation of 2) is something that can actually arise through miscommunication or simple inexperience.
In the case of 1) it may actually not be an issue in a game if everyone is having fun. However, in terms of GMing advice and scenario design it's usually better to focus on avoiding 1), because 1) is obviously the most common cause of 2)